Mixing circuit for electrical prospecting



AU@ 5, 1941. P. w. KLlPscH 2,251,549

MIXING CIRCUIT FOR ELECTRICAL PROSPECTING Filed Aug. 25, 1938 www Patented A141575, 1941 MIXING omcurr Foa ELECTRICAL rnosrso'rmo Paul W. Klipsch, Houston, Tex., assigner to E. E.

Rosaire, Houston, Tex..

Application August 25, 19438, Serial No. 226,669

12 Claims.

This invention relates to geo-electric prospecting with electric transients or impulses with steep wave fronts.

An object is to provide means for superposing an electric impulse of predetermined wave form upon a detected impulse so the wave form of the detected impulse may -be measured.

A more specific object isto provide such superposition means whereby the superposition is linear without the necessity of either impulse to pass through an amplier before mizdng.

A further objecteis to perform the mixing or superposition without interaction, that is without either wave having an effect upon or modifying the other.

An object is to provide synchronism between the source of the impulse with the detected impulse without radio or wire line communication and with negligible feed-back from the synchronized impulse to the synchronizing control potential.

Still another object is to lprovide a geo-electric prospecting system in which stray .capacitances, that would cause non-uniformity in performance, are avoided.

Other 'and further objects and advantages of the invention will -be apparent from the following description taken in connection with the single ilgure of the accompanying drawing which illustrates a form of apparatus for carrying out the invention.

Referring to the drawing reference numeral I represents an electric generator adapted to produce a current impulse of suitable wave shape to ilow in the earth Abetween electrodes 2 and 3. The generator I may be a grid controlled gas discharge tube such as a "'Ihyratron or Phanatron, supplied with direct current and a control potential whereby a sudden direct current impulse may be applied to the earth. `In a variation a switching means may be provided by additional tubes or relays to produce the impulses periodically, such as described by A. W. Hall in Physics IV, 1933, pp 66'I5.

Detection electrodes 4 and 5 serve to detect the potential produced by the current impulses, and thelwave form of the potential may be ob-l served and/or recorded by means of oscillograph f 'I after suitable amplification in repeater 6. In order to observe the wave form. it -is compared y with that of a locally generated impulse.

Generator 9 produces impulses of substantially network Ill may comprise any of various suitable forms, it is contemplated that this network shall preferably be constructed in accordance with the disclosure in copending application for patent by Klipsch et al. for Wave synthesizing network, Serial Number 226,668 med August 25. 1938.

In the present invention mixing or superpo sition takes place in a bridge circuit I2 before 'any amplication and attendant distortion has taken place. 'Ihe detected potential and locally generated. potential are connected in arms of a bridge with impedances which are shown as resistors I3 and Il forming the other two arms.

'I'he wave form indicating means comprising the repeater 6 and the recording means 1 connected across the diagonal between points 20 and 2i, as shown, or across the other diagonal between points 22 and 23 with the polarity of either potential reversed and will therefore indicate' a null when the detected and locally generated potentials have the same amplitude and wave shape and are in synchronism.

It should be noted that the bridge circuit I2 is a passive network in that no amplification occurs therein and that the elements thereof are linear since the current is proportional to voltage. 'I'he expressions passive" and linear" are conventional tothe art. T. E. Shea in "Transmission Networks and Wave Filters, Van Nostrand, 1928, Chapter 11, page 43, defines a passive network as one which contains no internal sources of power and a linear network as one whose circuit parameters do not vary with current. A discussion of linear elements also appears on pages 22, 23 of Operational Circuit Analysis" by V. Bush, John Wiley and Sons.

'I'he use of the bridge circuit as just described is highly desirable as such arrangement permits grounding of either or both the generator 9 andl the repeater 6 without introducing any disturbing influences that would obscure the results sought.

In order that generator 9 be in synchronism with the incoming potential, the latter may be used as a control potential acting through arepeater 25 and an electrical connection 26 to cohtrol the ignition of the discharge Atubes in generator 9, the repeater 25 being Vconnected* to electrodes 5 and 8. If the detectedpotential contains noise or other impulseswhich cause occasional Vmisflring and hence give rise to irregular` synchronization; the electrode' l. being nearer to the electrode 3 than .is-electrode I will increase the signal-tonoise ratio whereby -syn-v 2 9,351,549' l A chronization will have a small. lcontrollable lead or advance. If this rennement is not desired.

electrode 8 may be omittedand repeater 2l con-l r Y nected to' electrodes I and l. f l. l `In order to explain thel fact that no feed-back orinterlocking occurs it 'may be assumed that.

resistors il and Il are 500,000 each, that the eertn'e resistance between eieetrodes- 4 and l is l000'ohms and that the impedance looking.. into network Il from the left does not exceed 100i) ohms. When balance is approached, the'.

voltage output from the network Il is within a few percent of the potential detected betweenv If the network Il produces 10 miliivolts also.V

then approximately .0l millivolt will appear across the input to amplifier 25 due to potential from this network. This is only 0.1% of the detected potential and therefore does not contribute appreciably to the synchronizing' control'. In other words no interlocking will occur. It is to be Vunderstood that the foregoing values for resistances and voltages are illustrative only `and should not therefore be considered as limiting the invention. f

Control of synchronization by means of radio or by wire lines has been proposed. Certain inherent advantages of such means are o nsetl at least in part by serious dimculties. Inv case of radio, the transmitter induces several volts of radio frequency in the lines connecting the electrodes 4 and l with the indicating instrument, and in the presence of chemical contact between electrodes and ground, and rectification in the exists no stray. capacitances which would cause non-uniformity in' performance..since all apparatusmay bev grounded tothe same earth point.

-Theinve'ntion claimed-is: .Y

--1. of with varying electrical current which currenttoiiow inthe earth'spcrushdetecting the potential in a region subject to such-current.

e. 'mixing in a linearv passive network the detected amplifying system, several miliivoltsof rectifica tion (demodulation) products enterl the amplifier in such a way as to be superposed on the potential due to the current produced by the genera tor I. 'I'his results in errors of measurement both of steady state amplitude and of'wave form distortion.

In a similar manner wire lines,` consisting for as a single conductor with earth return to form potential.before -ampiiilcation thereof, with asynthetic potential. indicting the wave form of the syn' the mixed potentials, and maintaining thetic potential in synchronism withthcgenerated potential by' controlling the generation thereof with the amplied detected potential.

2. The methodof prospectingwith varying electric current comprising the steps fof 'causing a varying current to now inthe earth. detecting vthe potential between points in the area within which the current ilows, generating a potential similar in wave form to the wave form of the detected potential, luPerPOSinl said potentials, prior to the subiection of either potential to the influence of an active network, in a linear passive'network, and indicating the wave form of the resultant potential. A

3. The method of electrical prospecting which comprises. causing an electric current to flow in the earth's crust, detecting the potential between points in the area within which the `current flows, generating 'a potential similar in .wave form to the wave form ofthe detected potential. superposing such potentials. indicating the! wave form of the resultant potentiahand synchronizing the generated potential withthe detected potential by controlling the'generation thereof 4. In the method of geophysical prospecting wherein an electric current is caused to flow in the earth and potential is detected between spaced r points in the area of currentconduction, the example of a pair of wires from generator toref lupon the detected potential and .determining a line over which additional potentials ,wi1l be* produced between electrodes l and B vwhich have no relation to the potentials normally detected. The result is frequently a wave shape entirely dinerent from that which would have beenendepending upon the characteristics of thej activenetwork (amplifier), the state of battery voltages, the condition of the thermionic tubes in the amplifier and other factors.

Similarly,` if the detected potentiahthelocally generated potential and the repeater be interconnected in series or delta, capacitance'sbetween."

one earth point and another llenti toboth'. in

stability and errors in wave shape.

The amenities Just enumerated are eliminated in accordance with the teachings of the present invention which 'is also instrumental in vaccomplishing the stated objects aswell as additional objects apparent from the foregoing description directed to an embodiment of the invention. It is obvious that the bridge method of the` present invention is capable of keeping the capacitance across the detected potential-as low as the inputy capacitance of an amplifier `and falsothat the-rrv the' wave form of the resulting potential, the

step of synchronizing the second potential with the detected potential by controlling the generation thereof with a potential ybetween points which are differently spaced than are the points of detection. 1 5. In combination, means for passing electric impulses through the earth. means for detecting potential between spaced points in the area of current conduction, means for generating a potential in synchronismwith the detected potential, a bridge having said sources of potential constituting two armsl thereof, and indicating means` connected across one diagonal of the 6..'In combination, means for passing electric impuisesthrough the earth, means for detecting potential between spaced 'points in the area of current conduction, means .for generating a sec-v ond potentiaLa bridge having said sources of ypotential constituting two arms thereof, indicating means connected across one diagonal of the bridge,` and means operable from the detected potential to synchronize said generating means with generator and electrodes.` a pair of impedances, so that said potential sources with the impedances constitute the arms oi' said bridge, and indicating means connected across a diagonal of the bridge to indicate wave form of the superposed impulses from the generator and the detection electrodes.

8. In combination, means for causing intermittent current impulses to ilow in the earth, a generator, apair of detection electrodes embedded in the earth, a bridge comprising said generator, the earth between the potential electrodes and a pair of impedances, and indicating means connected across a diagonal of the bridge to indicate wave form of the superposed impulses from the generator and the detection electrodes.

9. In combination, means for causing a current impulse to flow in the earth, a generator, a pair of detection electrodes embedded in the earth, a bridge comprising said generator, the earth between the potential electrodes and a pair of impedances,- and indicatingmeans connected acrossa diagonal of the bridge to indicate wave form of the superposed impulses from the generator and the detection electrodes.

10. In combination, means for causing electric current impulses to ow in the earth, a generator, a pair of detection electrodes embedded in the earth to detect potential between points in the area of current conduction, a pair ot impedances, means interconnecting said generator, electrodes and impedances to form a bridge circuit, indieating means connected across a diagonal of the bridge to indicate wave form of the superposed impulses iiromv the generator and the detection electrodes, and means voperable from the potential between the detection electrodes to synchronize the generator with said rst mentioned means.

11. In combination, means for passing current impulses through the earth, means for detecting potential between spaced points in the region of current conduction, an impedance in series with said detection means, means for generating a potential in synchronism with the detected potential, a second impedance in series with said last mentioned means, means for connecting in parallel the detected and generated potentials and their series impedances. and across the so formed parallel circuit an indicating means.

12. An apparatus for geophysical prospecting comprising a generator adapted to feed current impulses into the earth: a pair of detection electrodes adapted to be arranged in spaced relation at a distance from said generator, a second generator of current impulses, a bridge including as adjacent arms the earth electrodes and the second generator, impedances arranged in the other adjacent arms of the bridge having impedance values high compared to the impedance values of the ilrst mentioned adjacent arms and indicating means connected across a diagonal of the bridge drawn between the points of intersection of the arms of each pair in such a way that the indicating means, one earth electrode and the second generating means all have a common ground.I

PAUL W. KLIPSCH. 

